Special nuclear material (SNM) is a term that refers to fissile materials which, in sufficient quantity, can be used to produce a nuclear weapon. The detection of SNM is an important part of international efforts to interdict such material that is being transported illicitly. To detect that activity, SNM monitoring systems are deployed at transportation portals where SNM might be conveyed. These systems use a variety of methods to detect SNM. One of the methods is gamma ray spectroscopy. It is very desirable to provide a means for checking the operability of such monitoring systems. However, a full-scale direct check of operability would require passing a large mass of SNM through the portal to test the monitor. Such a process is very undesirable because of the potential for diversion of the SNM test mass to rogue entities and the potential for accidental or terrorist-initiated dispersion of the material that might result in exposure of the public to hazardous material. What are needed therefore are devices that simulate comparatively large quantities of SNM and that can be used to check the operability of SNM monitoring systems without entailing the very undesirable risks involved with the use of large quantities of SNM.